For more than three quarters of a century, braking and actuation of clutches or other on-board power train devices in both on and off-road vehicles have typically been provided by hydraulic systems. In a hydraulic braking system, a brake actuation cylinder at each corner of the vehicle includes a piston that moves, in response to application of hydraulic pressure, to force a brake pad against a brake rotor or drum attached to one of the wheels of the vehicle, to slow or stop the wheel from turning. The brake cylinders at the wheels are connected via fluid conduits, known as brake lines, to a remotely located actuator, such as a master cylinder. The master cylinder includes a master cylinder piston that is moved in a pressure chamber of the master cylinder, when an operator steps on a brake pedal or moves a brake lever connected to the master cylinder, to generate hydraulic pressure for transmission through the brake lines to the brake cylinders.
Such a hydraulic braking system will also typically include a reservoir at the master cylinder, for supplying or receiving brake fluid from the brake lines as the pistons move in the master cylinder and wheel cylinders. Such systems also frequently include a booster, driven by engine vacuum, for augmenting the force applied by the operator to provide “power brakes.”
In recent years, hydraulic braking systems have also included sensors at the wheels to detect when the wheels are slipping, and devices for pulsing or reducing brake force under certain operating conditions to improve control of the vehicle. These systems are sometimes known as Antilock Brake Systems (ABS) or Traction Control Systems (TCS).
Many of the features and elements described above in relation to a vehicle braking system are also used for actuating hydraulic clutches, or shifting systems for transmissions, transfer cases, or other power train components in vehicles.
In all of the systems describe above, a central pressure producing device, such as a master cylinder, is used for producing hydraulic pressure that is transmitted via fluid conduits to the wheel cylinders, clutch, etc., in response to an operator directly actuating a piston in the pressure producing device by stepping on a pedal, or moving a lever.
There is a need and desire in some modern vehicles, however, to replace some of the components of a traditional hydraulic braking or actuation system with electrically operated components. Such a change is desirable, in some cases, to reduce cost or weight of the components themselves, and to improve space utilization in the vehicle, by allowing the master cylinder to be located remotely from the passenger compartment and actuated electrically, rather than being limited to arrangements in which the master cylinder is mechanically connected directly to a brake pedal or lever. Electrically operated components also offer the potential for improved control of the braking system, when coupled with an on-board computer capable of receiving inputs from on-board sensors and adjusting brake pressure applied at each wheel as a function of vehicle operating conditions. In general, brake systems using electrically actuated components are known as “brake-by-wire”(BBW) systems.
There is a need and a desire in some modern vehicles, to develop a force application system that does not rely on a central source of hydraulic pressure. In one form of such a non-centralized braking system, each wheel is equipped with a braking unit that includes an independent actuator for producing hydraulic pressure in a chamber of the actuator. The chamber pressure is connected via a brake line to a brake cylinder at the wheel. The brake cylinder operates in essentially the same fashion as the traditional hydraulic braking systems described above having a central source of hydraulic pressure for multiple wheels. The actuator at each wheel includes an electrical device, such as an electric motor, that is used to drive a piston in a bore of the actuator for generating pressure in the actuator chamber. The amount of braking force applied by the brake at each wheel is regulated by sending an electrical force actuation signal to the motor of the brake actuator. Such a system is commonly called a hybrid brake-by-wire (BBW) system, because, although the actual braking force applied to the wheel is still generated by hydraulic pressure acting on the piston of the wheel cylinder, the amount of pressure generated is regulated by electrical signals traveling to the braking units by wires rather than by fluid pressure traveling through brake lines or fluid conduits, as was the case in traditional hydraulic braking systems.
Regardless of whether the BBW system is a hybrid or not, it is generally desirable in BBW systems for the operator to perceive that the BBW system responds in very much the same manner, or in a superior manner, to a traditional hydraulic braking system. Achieving such performance presents significant challenges to designers of BBW systems, however, because the dynamic performance of a system including electrical components is fundamentally different from a traditional system having all hydraulic components, and a mechanical linkage between the driver and a central source of hydraulic pressure. Hydraulic systems have many linear and non-linear operating characteristics related to fluid volume, damping, pressure drops in the hydraulic circuit, and delayed operation of the components that are not found inherently in a BBW system. Electrically operated actuators can, in many instances respond to a signal input much more rapidly than hydraulic components, creating both design problems to be dealt with and opportunities to control performance that cannot be achieved in hydraulic systems.
In order to achieve a desired level of performance in a BBW system, commonly assigned United States patent application bearing the Common Assignee's docket number DP-305470, titled Gain Scheduling For Controlled Force Application and incorporated herein by reference, discloses a method and apparatus for determining and supplying an electrical force signal to an electrically operated actuator in response to a parameter of a desired force signal received from a brake pedal or other input signaling device. A controller receives the desired force signal, determines a parameter of the desired force signal, selects a set of gains based on the parameter, applies the set of gains to a linear control function, determines the electrical force control signal and sends the electrical force signal to the actuator. In this manner, a desired control performance is obtained through use of predetermined gain schedules and linear control functions in a manner that can be handled more effectively and efficiently by the controller than would be the case if the controller were simply programmed to incorporate a complex control function simulating both linear and non-linear characteristics of a typical hydraulic actuation system. Modern controllers are capable of handling a large number of gain schedules and linear control functions, thereby allowing even non-linear portions of a control spectrum to be simulated by a piece-wise simulation with linear equations.
Although the Gain Scheduling approach works well for controlling basic operation of a BBW system, there are special conditions such as an abrupt application and reapplication of the brake by the operator that must also be dealt with. Commonly assigned United States patent application bearing the Common Assignee's docket number DP-305469, titled Control Command Modification To Minimize Saturation Effects for Controlled Force Application and incorporated herein by reference, discloses including preset tables of gains related to a particular mode of operation, such as fast, normal or slow apply or release. Selection of a particular set of gains is made on the basis of inputs indicating that the force application system is operating in a particular mode of operation, as indicated by the difference between the current actuator pressure and the desired actuator pressure. In order to compensate for time lags inherent in a brake module, and to prevent conflicting signals from being sent to an actuator within the response time of a previously sent signal, a timing function is utilized. This timing function precludes saturation of the operational spectrum of the brake controller or overshooting the desired force output of the actuator as a result of operator actions, such as a rapid brake pedal position change during an apply mode.
Although the BBW systems described above work well for many types of controlled force actuation, in hybrid BBW systems having modules with high-speed actuators, such as motor driven ball screw piston devices for pressurizing hydraulic brake fluid to ultimately apply the brake, the problem of non-linearity of the system is exacerbated by the volatility of the hydraulic brake fluid. Under operating conditions known as fast mode release (FMR), where the piston is rapidly retracted by the actuator, resistance of the fluid to flow through the brake components and actuator is great enough to prevent the fluid from being pulled back from the force applying element as rapidly as the actuator can move the piston. As a result, the pressure of the hydraulic fluid drops so low within the actuator, for a short period of time during FMR, that volatile constituents of the hydraulic fluid may boil off, and form microscopic sized bubbles in the fluid that greatly reduce the modulus of the fluid.
In normal braking operations when the entrained microscopic bubbles are not present, the hydraulic fluid behaves in a predictable manner as a substantially incompressible fluid, such that any re-application of force by the actuator piston will be immediately transmitted to the force applying element. In contrast, fluid containing entrained microscopic bubbles behaves as a compressible fluid until the pressure in the actuator is increased to a point where the microscopic bubbles of volatile constituents are reabsorbed into the fluid. This can require very high pressures on the order of 100,000 pounds per square inch to cause re-absorption of the microscopic bubbles within a short enough time span to allow acceptable performance of the brake on reapplication of pressure by the actuator. Generating such pressures is typically beyond the desirable operational capability of the actuator device. At lower pressures, the time for re-absorption is too long for acceptable operation of the force generating apparatus.
During operation of a brake system for a vehicle under FMR conditions, microscopic bubbles in the fluid can even result in the actuator piston being damaged by striking the end of the cylinder, if the operator re-applies the brake too soon after releasing the brake. The time to reabsorb the microscopic bubbles can be far too long to allow safe operation of the vehicle, because the operator will not be able to re-apply the brake until the microscopic bubbles have been re-absorbed into the hydraulic brake fluid.
The present invention is aimed at resolving one or more of the problems identified above.